In a conventional ink-jet printer, sheet media is drawn through the printer via a feed mechanism which employs one or more feed rollers to direct sheets through the printer along a predetermined sheet media path. The rollers typically are configured to frictionally engage media within an input tray, consecutive sheets being pulled from the input tray and directed into a print zone where printing occurs. The rollers are moved with precision, promoting accurate placement of the sheet, and correspondingly, accurate placement of an image on the sheet.
Print quality thus is dependent on accurate identification of roller position, a task which typically is performed using a position encoder or the like. Such position encoders generally employ one or more sensors which identify incremental markings along a track of an adjacent code wheel (or code strip). The code wheel (typically a disc) is mounted for rotation with the feed roller, preferably about a central axis of both the feed roller and the code wheel. As the code wheel rotates, the sensor and attendant structure count the number of incremental markings which pass, each marking being indicative of a predetermined angular movement of the code wheel. Accordingly, it is possible to identify relative movements of the code wheel, of the rollers, and of the sheet which is carried thereby.
One problem with conventional position encoders has related to difficulties in identifying the absolute position (as opposed to relative position) of the code wheel. It will be appreciated, for example, that a conventional position encoder simply counts incremental changes in the position of the code wheel. It provides no reference by which the actual position of the code wheel may be identified. Such information, however, is important to addressing inherent encoder positioning errors (e.g. code wheel eccentricity), and correspondingly, is important to ensuring proper placement of an image on the sheet.
In the past, the aforementioned problem has been addressed by generating an index pulse for detection by an index pulse detector which is independent of the code wheel. The index pulse, it will be appreciated, is indicative of a predetermined position of the rollers and/or the to-be-printed sheet. Although potentially effective, this approach typically requires sophisticated optics, and requires that the index pulse be correlated to a precise position of the code wheel. These requirements increase complexity of the encoder and may be cost prohibitive in many commercial printer applications.
Another approach has been to add another track to the code wheel, and to add a corresponding channel to the position encoder. The position encoder thus is provided with a separate indexing subsystem which identifies a reference position of the code wheel. However, because the system employs separate indexing and incrementing subsystems, additional circuitry is required. Accordingly, this approach increases both the complexity of the position encoder, and the printer's overall cost.
What is needed is a position encoder system which is capable of identifying an absolute position of the code wheel without significantly increasing the complexity or cost over a conventional incremental encoder.